It is believed that the state of the art commercial high temperature piezoelectric actuators can be used at relatively low temperatures, for example up to about 200° C. The limitation is due to the piezoelectric material itself. For sensor applications there are some piezoelectric materials that can be used at relatively high temperatures, over 1000° C. in some cases, such as quartz and langasites, but these materials are non-ferroelectric piezoelectrics. Thus their piezoelectric coefficients are very low (<10 pm/V). For actuator applications, the displacements needed require piezoelectric coefficients in the order of hundreds of pm/V in order to build a stack that has a practical size for any actuator application. Ferroelectric piezoelectrics have large piezoelectric coefficients (d33) but are limited in their operation temperature.
Materials produced for large piezoelectric coefficients are based on Pb(Mg1/3Nb2/3)O3—PbTiO3 (PMN-PT) single crystals with d33>2000 pm/V in some cases. However, these materials can only be used up to about 70-90° C. Recent advances by addition of Pb(In1/2Nb1/2)O3 into PMN-PT, increased the operating temperature up to about 110° C. with minor decrease in piezoelectric coefficients. However, these temperatures are still low for several applications that require operating temperatures >300° C.
Materials proposed for high temperature piezoelectric are PZT based materials (Navy Type II) with Curie temperature (Tc) around 350° C. and d33 around 375 pm/V at room temperature. Curie temperature is the intrinsic limitation for the piezoelectric application above which the property does not exist. However, in most cases the limitation is due to de-poling and increased conductivity of the ceramics. For ferroelectrics to obtain a net piezoelectric effect, ceramics need to be poled by application of large electric fields which align dipoles in crystallographically allowed directions closest to the poling field. Upon removal of the field, most dipoles do not rotate back, resulting in net polarization and piezoelectric effect. The polarization (charge per unit area) in this case is due to bound charge and the ceramic stays insulating. However, when most ceramics are heated up, they become conductive and the strain inducing voltage drop across the material decreases. In addition, the temperature assisted motion of dipoles results in reorientation and randomization. This results in disappearance of a net piezoelectric effect. The electromechanical coefficients measured using the resonance peaks during impedance analysis decrease as the materials get de-poled. In some cases, especially for the ferroelectric materials with high Tc, de-poling happens at a lower temperature than Tc and becomes the limitation in operating temperature.
In view of the above, it would be desirable to provide piezoelectric ceramics that have various desirable properties. Methods for the production of piezoelectric ceramics are also described.